The role of coherent effects in mode bistability of a gas laser in inhomogeneous medium

We have studied microscopic effects leading to mode competition in an inhom ogeneously broadened medium of a gas laser with a weakly anisotropic cavity . The polarization bistability observed in some lasers, which is believed t o be caused by hole burning, we attribute to microscopic coherent effects i nduced by two light fields. Due to these effects one field can be coupled w ith another. Hole burning is found to be important, but not crucial. The ex act solution of the equation of motion for field amplitudes is compared wit h the third order approximation of perturbation theory. The role of four wa ve mixing, important for bistability of axial modes in a homogeneously broa dened medium, is discussed.